1. Technical Field
This invention is directed to hybrid opioid compounds, mixed opioid salts, and compositions comprising the hybrid opioid compounds and mixed opioid salts. Methods of use comprising administering an effective amount of the hybrid opioid compounds or mixed opioid salts to treat humans suffering from pain are also provided.
2. Description of Related Art
Opioid compounds remain key agents for the treatment of a wide variety of acute and chronic pain. The World Health Organization has recommended morphine as the analgesic of choice for the treatment of severe cancer pain. Additionally, morphine and related opioids are widely used to alleviate moderate to severe pain after surgery or trauma, or associated with medical illness such as heart attack. Patients with apparently similar pain states have large differences in opioid dosing requirements. Factors that contribute to this variability include psychosocial status, type of pain (nociceptive, inflammatory, neuropathic or mixed) and its severity, concurrent medications, gender and other genetic aspects, and whether patients are opioid-naïve or tolerant.
Unfortunately, the effects produced by morphine and similar opioid compounds make them amenable to abuse and are associated with many undesirable side effects, all mediated through activation of the mu (MOR) and other opioid receptors. They include physical and psychological dependence leading to addiction and other diverse pathophysiological states. Other undesirable side effects associated with the use of opioids include postoperative nausea and vomiting, drowsiness, respiratory depression and gastrointestinal and bladder dysfunction.
In addition to the adverse physiological effects listed above, a major associated risk is that repeated daily administrations of morphine or morphine-like opioids will eventually induce significant tolerance to the therapeutic effects of the drug as well as initiating some degree of physical dependence. Opioid tolerance is a phenomenon whereby chronic exposure to a drug diminishes its antinociceptive or analgesic effect, or creates the need for a higher dose to maintain its effect.
The degree of tolerance and physical dependence vary with the particular opioid employed, the correlation with morphine opioid receptor-selective opioids such as morphine being high, the frequency of administration, and the quantity of opioid administered.
In a wide variety of clinical indications requiring prolonged use of opioids, tolerance induction and addiction are closely linked, with the development of physical and psychological dependence always a major concern. Addiction with physical dependence can be difficult to treat due to the effects of withdrawal associated with dependence. Another undesirable effect of opioid tolerance is that the higher opioid requirements of highly tolerant patients treated for pain increase the likelihood of unpleasant non-analgesic side effects due to greater circulating concentrations of opioids and potentially toxic opioid metabolites (Smith, Clin. Exp. Pharmacol. Physiol. 2000, 27, 524-528; Ross et al., Pain, 1997, 73, 151-157).
The opioid receptor is thought to have four receptor subtypes named mu (morphine receptor), sigma (the phencyclidine receptor), kappa (the ketocyclazocine receptor) and delta (the endorphin/enkephalin receptor). The biochemical and cellular effects of morphine, including analgesia, are transduced through the mu opioid receptor (MOR), found in high concentrations within the central nervous system (CNS). The World Health Organization's guidelines for the management of chronic cancer pain recommend that clinicians reserve strong opioids such as oxycodone and morphine for the relief of moderate to severe cancer pain (World Health Organization, 1986) and that two strong opioids should not be co-administered, presumably because it is generally thought that all opioids exert their analgesic effects through the same receptor mechanisms in the central nervous system. However, recent studies by Maree Smith and co-workers have shown that the antinociceptive effects of structurally related oxycodone and morphine are differentially antagonized by nor-BNI (a κ-selective opioid antagonist) and naloxonazine (selective μ-opioid receptor antagonist), indicating that they produce antinociception through different opioid receptor mechanisms (see Ross et al., Pain 1997, 73, 151-157). Furthermore, it has been found that co-administration of sub-antinociceptive doses of oxycodone with morphine to rats resulted in synergistic levels of antinociception (Ross et al., Pain 2000, 84, 421-428). Importantly, it was found that animals that received the sub-antinociceptive doses of oxycodone and morphine were similar to control animals with respect to CNS side effects. Administration of equipotent-doses of either opioid alone resulted in sedation of the rats. This may suggest that co-administration of sub-analgesic doses of oxycodone and morphine to patients may provide synergistic antinociceptive relief with a reduction of CNS-related side effects.
One of the most challenging aspects of the treatment of infectious disease is the development of drug-resistant strains of the infectious agent. Disease-causing microbes that have become resistant to drug therapy are an increasing public health problem. Tuberculosis, gonorrhea, malaria, and childhood ear infections are just a few of the diseases that have become hard to treat with antibiotic drugs. The widespread development of multi-drug resistant forms of malaria in Africa and South East Asia is one such troubling phenomenon. The protozoal parasite responsible, plasmodium falciparum, has gained resistance to most forms of monotherapy, including chloroquine, a cheap and effective antimalarial drug that has been used for more than 40 years.
The scientific community has been actively developing new drugs to combat the increasingly drug-resistant strains of these and other infectious agents. One interesting approach to fight drug-resistant strains is the development of hybrid drugs that combine active agents with independent modes of action. Using this strategy, new active agents have been prepared that show much promise for the treatment of resistant microbes. For example, Walsh and co-workers prepared novel hybrid molecules comprising active components of the drugs artemisinin and quinine. Walsh et al., Bioorg. Med. Chem. Lett., 2007, 17(13), 3599. The hybrid drugs were reported to have potent activity against 3D7 (drug resistant) and FcB1 strains of Plasmodium falciparum in culture. The activity was found to be superior to artemisinin and quinine alone. Dechy-Cabaret et al., (Chembiochem, 2000, No. 4, 281-283) reported the preparation of a novel trioxaquine molecules that contain that combine the peroxide entity of the trioxane-containing drug artemisinin with an aminoquinoline group related to chloroquinine that is known to penetrate into infected erythrocytes. The resulting hybrid drug was found to be highly active against chloroquinine-resistant strains. Burgess et al. reported the preparation and evaluation of hybrid drug molecules designed to include components of the drug chloroquinine and a pharmacophore that is known to inhibit chloroquinine resistance. Burgess et al., J. Med. Chem. 2006, 49, 5623-5625. The hybrid compounds were found to inhibit the growth of P. falciparum (resistant to chloroquinine) in vitro and after oral dosing in vivo.